Best Regular Expression for a one or two values optionally divided by a certain character

Question

I'm looking for the most concise regex that matches one or two 4-digit years in any the following setups:

• year
• year-
• -year
• year-year

I can't think of anything slicker than this:

[\\-]?\d{4}|\d{4}\[\\-](\d{4})?

Answer 1

I am assuming you prefer the longest match. That is, if the input line is:

xyzzy 2000-2010 xyzzy

then matching 2000 or 2010 or 2000- or -2010 is not what you want, even though these would be valid matches the way you have stated the problem.

In Perl 5.10 and later, you can reduce the pattern to 20 characters:

(\d{4})-(?1)?|-?(?1)

Let's break this down.

(\d{4}) # match a year and capture the pattern
-       # match a hyphen
(?1)?   # match a year again if possible
  |     #   OR,
-?      # match an initial hyphen if possible
(?1)    # match a year

Things get more complicated if you prefer to match two years even in cases such as:

xyzzy -2000-2010 xyzzy

See: http://perldoc.perl.org/perlretut.html#Recursive-patterns

Answer 2

This is a somewhat difficult one because regular expressions inherently lack memory, so you can't tell on the back whether the front existed, so I don't think one can get better than the one you wrote for that particular set. If you wanted to allow some sort of variant, you could potentially find a better one.

Answer 3

If you first removed all "-" characters you could make it

(\d{4}){1,2}

or

(\d{4}|\d{8})